1. Field of the Invention
The present invention relates to a radial type dynamic pressure fluid bearing (also commonly known as a xe2x80x98hydrodynamic bearingxe2x80x99), and particularly to a foil type fluid bearing that includes a stationary retaining member that surrounds the outer circumference of a rotating journal so as to form an annular gap therebetween and a foil assembly which is positioned in the gap and supports the journal.
2. Description of the Related Art
A conventional foil type fluid bearing is known in, for example, U.S. Pat. Nos. 4,415,280, 5,902,049, and 5,988,885.
FIG. 12 shows a basic structure of the above-mentioned conventional foil type fluid bearing B. The foil type fluid bearing B includes a cylindrical stationary retaining member 02, and a foil assembly disposed in a gap between a rotating journal 01 and the retaining member 02 and supporting the journal 01. The foil assembly includes, going from the inside to the outside in the radial direction, an annular top foil 03, an annular mid foil 04, and corrugated bump foils 05.
Radial displacement of the rotating journal 01, which is supported by the fluid pressure within the foil assembly, generates frictional damping forces on the sliding faces of the top foil 03 and the mid foil 04, on the sliding faces of the mid foil 04 and the bump foils 05, and on the sliding faces of the bump foils 05 and the stationary retaining member 02, thereby suppressing vibration of the journal 01. However, since the above-mentioned conventional arrangement employs only one mid foil 04, it is difficult to generate a sufficient level of frictional damping force, leading to a possibility that the journal 01 might undergo a damaging resonance phenomenon.
The top foil 03 positioned in the innermost layer of the foil assembly receives a radially outward load from the fluid that is present between the top foil 03 and the journal 01, and this load is transmitted from the top foil 03 to the stationary retaining member 02 via the mid foil 04 and the bump foils 05. Since the corrugated bump foils 05 support the outer circumference of the mid foil 04 at a plurality of support points 5a, which are arranged in the circumferential direction at fixed intervals, in some cases the above-mentioned radially outward load causes the top foil 03 and the mid foil 04 to bend radially outward between the above-mentioned plurality of support points 05a. In this way, a wavelike deformation of the top foil 03 and the mid foil 04 not only prevents sliding, which generates the frictional damping forces, between the foils, but also makes it difficult to form a wedge shape, which generates a pressure to float the journal 01, between the top foil 03 and the journal 01, and there is therefore a possibility that the function of the foil type fluid bearing B might be adversely affected. FIG. 12 illustrates the deformation of the top foil 03 and the mid foil 04 in an exaggerated manner.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to generate a sufficient level of frictional damping force in a foil assembly of a foil type fluid bearing, and to prevent a top foil from being deformed in a wavelike manner due to the pressure of the fluid that is present between a journal and the top foil.
In order to accomplish the above-mentioned object, in accordance with a first aspect of the present invention, there is proposed a foil type fluid bearing that includes a stationary retaining member surrounding the outer periphery of a rotating journal so as to form an annular gap therebetween, and a foil assembly disposed in the gap and supporting the journal. The foil assembly includes a top foil, a mid foil section, and a corrugated bump foil disposed between the mid foil section and the stationary retaining member. One end of the top foil that faces a slit formed in the axial direction is fixed to the stationary retaining member. The top foil is wound in a cylindrical manner from the one end to the other end and supports the outer circumference of the journal via a fluid film layer. One end of the mid foil section that faces a slit formed in the axial direction is fixed to the stationary retaining member and the mid foil section being wound in a cylindrical manner from the one end to the other end. The inner circumference of the mid foil section is in frictional contact with the outer circumference of the top foil in a slidable manner, thereby generating a frictional damping force that counteracts a radially outward movement of the journal. The mid foil section includes at least two mid foils, which are superimposed on each other in a slidable manner so as to be in frictional contact with each other.
In accordance with the above-mentioned arrangement, with regard to the foil type fluid bearing in which the foil assembly including the top foil, the mid foil section and the bump foil is disposed between the stationary retaining member and the journal. Since the mid foil section includes at least two mid foils superimposed on each other in a slidable manner so as to be in frictional contact, when the journal moves in the radial direction, a large frictional damping force can be generated between the plurality of the mid foils, thereby enhancing the function of supporting the journal. Moreover, since the plurality of the mid foils are superimposed on each other, the rigidity against a load in the radial direction is enhanced, thus preventing the top foil from being deformed in a wavelike manner and thereby allowing the function of the foil type fluid bearing to be exhibited reliably.
Furthermore, in addition to the above-mentioned first aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein the plurality of the mid foils forming the mid foil section are all wound in the same direction.
In accordance with the above-mentioned arrangement, since the plurality of the mid foils are all wound in the same direction, they can be integrated into an assembly in advance and mounted together in the stationary retaining member, thereby contributing to a reduction in the number of assembly steps.
Furthermore, in addition to the above-mentioned first aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing, wherein at least one mid foil among the plurality of the mid foils forming the mid foil section is wound in the opposite direction to that in which the other mid foils are wound.
In accordance with the above-mentioned arrangement, since at least one mid foil among the plurality of the mid foils is wound in the opposite direction to that in which the other mid foils are wound, it is possible to increase the amount of sliding between the mid foils that are wound in different directions, thereby generating a higher level of frictional damping force.
Furthermore, in addition to the above-mentioned aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein the mid foil in the innermost layer among the plurality of the mid foils forming the mid foil section is wound in the same direction as that in which the top foil is wound.
In accordance with the above-mentioned arrangement, since the mid foil in the innermost layer and the top foil are wound in the same direction, they can be integrated into an assembly in advance and mounted together in the stationary retaining member, thereby contributing to a reduction in the number of assembly steps.
Furthermore, in addition to the above-mentioned aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein the mid foil in the innermost layer among the plurality of the mid foils forming the mid foil section is wound in the opposite direction to that in which the top foil is wound.
In accordance with the above-mentioned arrangement, since the mid foil in the innermost layer is wound in the opposite direction to that of the top foil, it is possible to increase the amount of sliding between the mid foil and the top foil, which are wound in different directions, thereby generating a higher level of frictional damping force.
Furthermore, in addition to any one of the above-mentioned aspects, in accordance with yet another of the present invention, there is proposed a foil type fluid bearing wherein at least one part of the mid foil in the outermost layer among the plurality of the mid foils forming the mid foil section is formed from a material that is thicker than that used for the other mid foils.
In accordance with the above-mentioned arrangement, since at least one part of the mid foil in the outermost layer is formed from a material that is thicker than that used for the other mid foils, it is possible to prevent the top foil from being deformed in a wavelike manner due to the load from a fluid pressure, thereby allowing the function of the foil type fluid bearing to be exhibited reliably.
Furthermore, in addition to any one of the above-mentioned aspects, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein at least one part of the mid foil in the outermost layer among the plurality of the mid foils forming the mid foil section is formed from a material that is harder than that used for the other mid foils.
In accordance with the above-mentioned arrangement, since at least one part of the mid foil in the outermost layer is formed from a material that is harder than that used for the other mid foils, it is possible to prevent the top foil from being deformed in a wavelike manner due to the load from a fluid pressure, thereby allowing the function of the foil type fluid bearing to be exhibited reliably.
Furthermore, in addition to any one of the above-mentioned aspects, in accordance with yet another aspect of the present invention, there is proposed a foil type fluid bearing wherein at least one area of the inner circumference of the top foil is coated, and the coated area has a coefficient of friction that is lower than that of any of the other sliding faces of the foil assembly.
In accordance with the above-mentioned arrangement, since the coefficient of friction of the coating applied to said at least one area of the inner circumference of the top foil is lower than that of any of the other sliding faces of the foil assembly, it is possible to start rotation of the journal smoothly and prevent the occurrence of any abnormality such as burning.
Furthermore, in addition to the above-mentioned aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein the coating is carried out using diamond-like carbon (DLC).
In accordance with the above-mentioned arrangement, since at least one area of the inner circumference of the top foil is coated using DLC, it is possible to reliably prevent damage to the inner circumference of the top foil due to friction between the top foil and the journal.
Furthermore, in addition to the above-mentioned aspect, in accordance with another aspect of the present invention, there is proposed a foil type fluid bearing wherein the coating is carried out using Polytetrafluoroethylene (PTFE).
In accordance with the above-mentioned arrangement, since at least one area of the inner circumference of the top foil is coated using PTFE, it is possible to reliably prevent damage to the inner circumference of the top foil due to friction between the top foil and the journal.
Furthermore, in addition to any one of the above-mentioned aspects, in accordance with yet another aspect of the present invention, there is proposed a foil type fluid bearing wherein a metal layer coating is applied to at least one area of the inner circumference of the stationary retaining member and the sliding faces of the foil assembly other than the inner circumference of the top foil.
In accordance with the above-mentioned arrangement, since a metal layer coating is applied to at least one area of the inner circumference of the stationary retaining member and the sliding faces of the foil assembly other than the inner circumference of the top foil, the frictional damping force of the foil assembly can be increased.